Initially, perhaps even unknowingly, the young Mexican Dominican Manuel Aguas was drawn to the path of Martin Luther. Like the German theologian, Aguas read the Bible and his ruminations convinced him to break with the Roman Catholic Church. In response, the ecclesiastical institution excommunicated him. At the heart of this chapter is a letter in which Manuel Aguas provides an account of his conversion to Protestantism. The account caused a great commotion in Mexico City. Aguas's writing was published in El Monitor Republicano on April 26, 1871. Despite the influence of Aguas’ ideas, there is no doubt that he benefited from the past efforts of various converts that attempted to establish Protestantism in Mexico City. In this sense, he fertilized a ground prepared by others but added an activism that, within a few months, garnered public attention for the challenges it posed to the religious and cultural establishment of the time. His account makes visible the construction of a marginalized faith through his vigorous attempts to defend its legitimacy in an environment that overtly denied it.